1. Field of the Invention
The present invention relates to a radiographic apparatus which obtains image information by detecting X-rays that pass through a subject with an image capturing unit having an X-ray detector mounted therein.
2. Description of the Related Art
In general, a radiographic apparatus includes an image capturing unit disposed at a position spaced from an X-ray generator and having an X-ray detector mounted therein. An image of a subject, such as a human body, is captured while the subject is placed between the X-ray generator and the image capturing unit. A required image is obtained by emitting X-rays from the X-ray generator after the image capturing unit is positioned relative to the X-ray generator and the subject.
Recently, the processing speed of image processing apparatuses has considerably increased and apparatuses for capturing not only two-dimensional still images but also three-dimensional images and moving images have come into widespread use.
The three-dimensional images have generally been captured using computed tomography (CT) apparatuses. However, recently, a method for obtaining three-dimensional images as a result of calculation based on two-dimensional images using a two-dimensional sensor such as an image intensifier (I.I.) or a flat panel detector has also been used.
In an image capturing method using a C-arm image capturing apparatus, an X-ray generator and an image capturing unit are supported by a C-arm and are rotated around a subject lying on a subject support, such as a bed, so that X-ray projection images of the subject can be obtained at different angles.
Japanese Patent Laid-Open No. 2000-116635 describes a radiographic apparatus which captures X-ray projection images of a subject at different angles by emitting X-rays from an X-ray generator while a subject standing on a turntable is rotated. Since a relative movement occurs between the image capturing system and the subject, multiple two-dimensional X-ray projection images are obtained. The thus-obtained X-ray projection images are used to obtain a three-dimensional image by calculation.
To obtain clear images, it is important to position the X-ray detector included in the image capturing unit as close to the subject as possible. In addition, to obtain an image that covers a large area of the subject, it is necessary to increase the area of the X-ray detector. Therefore, the size of the image capturing unit that includes the X-ray detector increases accordingly. As a result, in the image capturing apparatus which moves the image capturing system and the subject relative to each other, there is a risk that a part of the image capturing unit will come into contact with or collide with the subject. In such a case, there is a possibility that the relative movement required in the image capturing operation will be blocked or the subject will be injured.
Accordingly, a radiographic apparatus having a safety mechanism including a moving unit and a detecting unit for minimizing the damage is known. In this apparatus, an impact applied when the image capturing unit collides with the subject is suppressed and movements of movable components are stopped when a contact is detected.
Japanese Patent Publication No. 3-561 describes an apparatus including a contact detection mechanism, which is an example of a safety mechanism. In this type of apparatus, it is assumed that the image capturing operation is performed while the subject and the image capturing unit are not in contact with each other.
On the other hand, the most common use of the radiographic apparatus is to obtain a two-dimensional still image of, for example, an appendage like a breast of a human body in a still-image capturing operation. In the still-image capturing operation, the subject is positioned as close to the image capturing unit as possible and the operation is performed while the breasts, the chin, etc., of the subject are in contact with an outer cover portion of the image capturing unit. Thus, the detection area of the X-ray detector can be maximally used and high resolution images can be obtained.
Japanese Patent Laid-Open No. 2000-205082 describes a radiographic apparatus which emits X-rays while rotating a subject standing on a turntable. This radiographic apparatus has a first image capturing mode in which X-ray projection images are obtained from all directions in the range of 360° around the subject and a second image capturing mode in which a two-dimensional still image of the subject is obtained.
The above-described radiographic apparatus including the image capturing unit provided with the safety mechanism including the moving unit and the detecting unit has the following problem. That is, in the image capturing operation performed while the subject and the image capturing unit are in contact with each other, it is difficult to position the subject and the image capturing unit with respect to each other.
To solve this problem, a radiographic apparatus has been proposed in which a contact detection device including a moving unit and a detecting unit can be detachably attached to an image capturing unit. According to this apparatus, the positioning process can be facilitated.
However, the size of the contact detection device including the detecting unit is large enough to cover most of the image capturing unit so that contact or collision can be detected in a large area. In addition, since the contact detection device must be sufficiently rigid, the weight thereof is large. Therefore, it is very burdensome for an operator to attach and detach the contact detection device.
In addition, a space, a shelf, etc., is necessary for temporarily storing the contact detection device while it is not used. Therefore, in facilities that are not very large, there is a problem that space cannot be efficiently used.
In addition, the contact detection device is a precision device, and is easily damaged if it is roughly handled. Therefore, the contact detection device must be handled with care in the process of attaching or detaching the contact detection device or after it is detached. Thus, a burden is imposed on the user. In addition, an external body having a double-layer structure must be formed and the number of components is increased accordingly. Therefore, high costs are incurred.